1. Field of the Invention
The present invention relates generally to the Global Positioning System (GPS) and more particularly, to GPS receivers and GPS antenna systems.
2. Description of the Related Art
The GPS includes a constellation of 24 satellites that are positioned with four satellites in each of six orbital planes. The satellite orbits are nearly circular and their orbital planes are inclined from the equator by 55.degree. and spaced 60.degree. apart at the equator.
Each GPS satellite transmits right hand circularly-polarized (RHCP) signals at two carrier frequencies--L1 at 1575.42 MHz and L2 at 1227.6 MHz. The carriers are modulated by navigation data and by ranging codes. The latter are spread spectrum codes having a unique pseudorandom noise (PRN) sequence associated with each satellite. With the navigation data, a GPS receiver determines the satellite's location at the time of signal transmission and with the ranging codes, it determines time and the satellite-to-receiver range and velocity.
In particular, the navigation data includes updated information on the satellite's orbit (e.g., eccentricity, inclination and mean anomaly) so that a GPS receiver can accurately determine satellite location. To utilize the ranging codes, the receiver replicates the PRN sequence of a received signal and time shifts this sequence in a code tracking loop until it correlates with the received sequence. The required time shift is indicative of the distance between the receiver and that satellite.
Typically, the receiver also determines its velocity by processing carrier phase in a carrier tracking loop to detect Doppler frequency shifts and thereby, the receiver-to-satellite velocity. Although the receiver's clock generally has an offset from the GPS system time, redundant information from several satellites (generally four or more) can be processed by least squares estimation techniques to identify the offset and reduce its effect.
Two PRN ranging codes are transmitted from the GPS satellites; a "short" coarse/acquisition (C/A) code having a 1,023 chip sequence length, a 1.023.times.10.sup.6 chip rate and a resultant 1 millisecond period and a "long" precision (P) code having a 6.1871.times.10.sup.12 chip sequence length, a 10.23.times.10.sup.6 chip rate and a resultant seven day period. Received GPS signal levels generally do not exceed -123 dBm and -125.5 dBm for the C/A code and P code components on the L1 channel and -128 dBm for either code on the L2 channel. Although these GPS signals are below (e.g., .about.20 dB) thermal noise in the bandwidth of a typical GPS receiver, they can be acquired and tracked because processing gains of the C/A and P codes are respectively on the order of 43 and 53 dB.
GPS signal acquisition and tracking becomes more difficult, however, when the GPS receiver is subjected to interference signals. These signals can be unintentional (e.g., radio, television and radar transmissions) or intentional (e.g., wideband-Gaussian and spread spectrum jammer signals and narrow-band swept jammer signals). A GPS receiver in an advanced missile guidance system, for example, may be threatened by intentional jammers whose interference signals result in receiver failure or unreliable tracking (e.g., missing synchronization in the code tracking loop).
Various responses have been proposed to interference signals. In an exemplary one, U.S. Pat. No. 5,712,641 provides antenna structures that decompose a GPS signal into two orthogonally-polarized signals which are then processed through a two-stage polarimeter. In each polarimeter stage, each input signal passes through its own phase shifter and the output signals are combined in a 90.degree. hybrid. The delta port of the final 90.degree. hybrid generates a difference signal which is passed on to conventional GPS receiver circuits and is also processed in a control loop to adjust the phase shifters so as to null the signal at the delta port.
In an exemplary adjustment process, one phase shifter of the second stage is set to five different settings and, for each setting, the other second-stage phase shifter is stepped over a 180.degree. range. The settings that produce the least delta signal are selected and this entire process is then repeated with the first-stage phase shifters. Subsequently, one second-stage phase shifter is fine tuned in several steps over a narrow range and left at the setting that achieved the lowest delta signal. This fine tuning process is then repeated with one first-stage phase shifter. The narrow range is further narrowed and the entire fine tuning process is repeated to again reduce the delta signal.
In U.S. Pat. No. 5,694,416, an exemplary antenna array has four antenna elements and a downconverter structure (e.g., a serial connection of a pre-selector, two mixers, bandpass and lowpass filters, an ADC and a digital equalizer) dedicated to each antenna element. Five tracking channels are each coupled to each of the equalizers. Each tracking channel includes code despreaders and I/Q accumulators for carrier and code tracking.
At baseband in the fifth channel, an interference data generator receives non-despread but accumulated signals and forms an estimate of the non-despread but accumulated interference data. This data is used in correlators to spread interference energy while coherently adding satellite energy over a correlation interval.
U.S. Pat. No. 5,347,284 describes antenna, downconverter, ADC and baseband processors in which null zone processing is included in the ADC to provide a degree of immunity to continuous wave (CW) interference signals. In particular, threshold detectors sense the portions of time the downconverted and digitized signals spend at low, average and high thresholds. Because a CW signal spends a larger portion of time near the low and high thresholds than spread spectrum signals, its presence can be sensed and, in response, weights applied to the digitization process to attenuate low and high signals and, thereby, reduce CW signal strength.
Although the systems described above may enhance the ratio of GPS-to-interference signal strength, the polarimeter of U.S. Pat. No. 5,712,641 is a complex microwave structure and its alignment process is lengthy and time-consuming, the approach of U.S. Pat. No. 5,694,416 is hardware intensive for it requires a receiver downconverter for each antenna element and applications of the teachings of U.S. Pat. No. 5,347,284 are limited to CW interference signals.